1. Field of the Invention
The present invention relates to a method of driving a direct current (DC) brushless motor and an apparatus for the same and, more particularly, to a method and an apparatus for driving a DC brushless motor (or brushless DC motor) while controlling the starting position of a rotor of the motor. The method and apparatus to which the present invention pertains are advantageously applicable to a printer and like machines.
2. Description of the Prior Art
A DC brushless motor known in the art includes a rotatable rotor having a disk-like magnet having N and S poles arranged alternately around a shaft of the rotor, and a stator provided with a plurality of armature coils which are arranged around a bearing of the stator, which supports the shaft of the rotor, in such a manner as to face the disk-like magnet. This kind of DC brushless motor is disclosed in, for example, U.S. Pat. No. 4,631,457 which was assigned to the same assignee as the instant application. In the motor disclosed, a Hall generator is disposed at a central part of each of particular ones of the armature coils which are held in a predetermined relationship in terms of electrical angle. A circuit pattern for the detection of rotor speed is provided to surround the armature coils and with the bearing at the center. An annular magnet having N and S poles arranged alternately on a circle is fitted on the rotor in face-to-face relation to the circuit pattern on the stator, cooperating with the circuit pattern to constitute a frequency generator. A drive circuit associated with the motor responds to the frequency generator and Hall generators by switching drive currents applied to the armature coils, whereby the motor is controlled for acceleration, deceleration and constant-speed rotation. A drawback with such a prior art DC brushless motor is that it needs a great number of circuit parts and elements and, in addition, suffers from the scattering in the performance of the Hall generators as well as from errors inherent in an assembly line. Another drawback is that the Hall generator which is located at the center of each particular armature coil limits the number of turns which may be provided on the inner side of the coil and, thereby, the output torque of the motor while rendering the assembly complicated.
To eliminate the drawbacks discussed above, the assignee of the instant application has proposed a DC brushless motor driving method which implements a control with a motor speed sensing device only and without resorting to those devices which are sensitive to the angular positions of a rotor, i.e. Hall generators (see Japanese Patent Laid-Open Publication No. 194692/1984). A DC brushless motor to which this driving method is applicable is not provided with Hall generators. A motor speed sensor plate is rigidly mounted on a rotor in place of the annular magnet. This plate is provided with a number of slits which are arranged on a circle the center of which is defined by a shaft of the rotor. A pair of speed sensors are located to face the slits of the speed sensor plate with their phases deviated by an electrical angle of 90 degrees from each other. In this arrangement, the speed sensors produce pulses the frequency of which corresponds to the rotation speed of the rotor. A control circuit adapted to drive the motor switchs drive currents applied to armature coils in response to the output pulses of the speed sensors, thereby controlling the acceleration, deceleration and constant-speed drive of the motor. At the time of starting the motor, currents fed to the armature coils are switched to excite them over several phases so as to locate the rotor at a predetermined starting position. Thereafter, output pulses of the speed sensors are counted to determine each instantaneous angular position of the rotor and, thereby, to control the rotation of the motor.
In the event of start-up, the rotor will be located at the starting position with accuracy if the currents applied to the armature coils are switched in a predetermined sequence so as to excite the coils over several phases. To reverse the rotation of the rotor, a current in the direction of a certain phase is applied to the armature coils to continue the rotation of the rotor and, when a predetermined phase is reached, a command indicative of the direction of current of the next phase and a command indicative of the opposite direction are prepared. As the torque exerted by the current flowing through the armature coils to the rotor becomes zero to stop the rotor, the current application command in the opposite direction is delivered to supply that current to the armature coils. Such a procedure allows the rotor to be located at the starting position without the need for Hall generators.
However, as will be understood from the above, the rotor torque becomes substantially zero when the rotation direction of the rotor is reversed. Hence, should any load act on an apparatus which is connected to the output shaft of the rotor to be driven thereby, the rotor might fail to be stopped at its ideal stop position. In a printer, for example, a mechanism for transporting a carriage which is loaded with a print head is connected to the output shaft of the motor. Usually, the print head is located to face a paper sheet with the intermediary of an ink ribbon, so that friction is generally developed between the print head and the ink ribbon. In this situation, it often occurs that when the rotor torque is reduced to substantially zero in the vicinity of the stop position, the rotor is prevented from being brought to a halt accurately at its starting point due to the frictional load. Stated another way, the load which is constituted by the ink ribbon is apt to prevent the rotor from being positioned at the predetermined stop point with accuracy. This positioning error invites relatively great ripples during motor drive to follow and, thereby, makes constant-speed drive difficult.